The incident occurred during a live broadcast on Monday night.
A woman entered the studio during an ongoing news broadcast and held up a sign behind the newscaster.
The sign read, among other things, "stop the war" and "do not believe in propaganda".
After the woman was seen in the picture for a few seconds, a pre-recorded news item started.
In a clip that the woman posted on social media, she says that what is happening in Ukraine is a crime.
- Unfortunately, I have worked on Kanal 1 for the past year and I am very ashamed of it.
I teach the Kremlin's propaganda, the woman says.
Must have been arrested
The woman is said to work as an editor on the TV channel and has been arrested by police, according to the Russian state-controlled news agency Tass.
The regime-loyal TV channel is now investigating the incident, according to the same source.
After the invasion, the Kremlin has tightened the thumbscrews for the country's media.
Among other things, they have been banned from calling the war in Ukraine a war.
Instead, the media is forced to call it a military special operation.
A newly enacted law means that anyone who reports in a way that "discredits" the Russian military risks imprisonment or a large fine.
According to Tass, the woman is suspected of violating the new law.